While various features of the invention are applicable to vessels of various sizes adapted for various uses, military, commercial and pleasure, the invention will be described as specifically exemplified in a 48 foot, twin diesel engine, cruiser yacht capable of planing speeds. Typical prior art yachts of this size are powered with twin diesel engines located near the longitudinal center of the hull. The space above the engines is almost always utilized for the salon or living room area, which sometimes includes the galley. There is usually an owners cabin or a stateroom located aft of the engines and one or two guest staterooms forward. The floor level of the salon is relatively high due to the space required by diesels of appropriate 250-450 horsepower size. This in turn requires that the control station for operating the boat be located aft of the salon, which generally results in poor forward visibility, or on a flybridge above the salon. Due to the relatively high location of such a flybridge over the salon, it is not usually feasible to fully enclose this for weather protection because the height of the boat and the extra topside weight would be excessive. Hence two control stations are generally provided; an open one above and an enclosed one below. In order to keep the overall height down as much as possible, the salon floor is as close as possible to the top of the engines. This makes access to the engines and other machinery located in this area, such as AC generating plant, air conditioning compressors, hot water heater, pumps, and the like, very difficult for routine maintenance or overhaul. Any major work requires opening large hatches in the salon floor which disrupts and dirties the living area. In normal operation, all of the noise, fumes, heat and vibration of this machinery are located a few inches below the feet of the occupants of the salon. In addition, the heavy weight of these diesel engines usually requires that other major weight, such as fuel and water, be placed aft, such as under the bunks and cabinets. The variation of the weights of the fuel and water in the tanks as the liquids are consumed causes frequent problems with the trim angle of the boat. It has been suggested that a better arrangement is to place the fixed engine weight well aft and the variable fuel weight forward near the middle of the boat where its change has less effect on the trim angle. In addition, if there is an open cockpit at the stern with the engines under this floor, the noise and heat are moved away from the primary living area and hatches can be provided in the cockpit floor for easy access. This arrangement has been tried occasionally using vee drive gear boxes. With these, the engine is mounted as far as possible to the stern with the output shaft pointing forward; a special gear box then turns the power train in a "V" to connect the engine to the propeller shaft and propeller extending aft. While this is satisfactory in theory, the cost, weight and mechanical complexity of vee drive gear boxes for large, high-torque diesel engines causes problems and such arrangements have never become popular. In addition, this moves the engine weight so far aft that some of the machinery must be located forward to compensate. Hence the AC generating plant has usually been placed under the salon in these arrangements, defeating much of the original objectives.
It has been known to provide partial propeller tunnels in order to locate engines somewhat further aft than usual, as, for example, in Hatteras 31, SeaCraft 23 and Phoenix 29 gasoline engine cruisers, and tunnels have also been employed to provide for shallow draft operation for small boats, as shown in U.S. Pat. Nos. 3,515,087 and 3,611,973, for example.